We propose a role sharing model of language areas in which Broca's area is for categorizing symbols used to represent rules stored and retrieved in other language areas. For example, at the syntactical level, the other language areas store rules represented with terminal symbols and also rules represented with non-terminal symbols, whereas Broca area invents non-terminals and forms abstract rules at language acquisition phase and converts a terminal symbol to a non-terminal corresponding to it to get an appropriate rule at performance phase. The model role of Broca's area is supposed to be essential but minimal to support human language faculty. Under this assumption, the emergence of Broca's area is hypothesized to have triggered the evolution of language supposing that the other mechanisms are fully evolved. The argument is based on the recent fMRI study of KE family members and related findings on a mutation of FOXP2 gene.